#include<stdio.h>
#include<math.h>
int main()
{
	float x1, x2, y1, y2;
	scanf_s("%f%f%f%f", &x1, &y1, &x2, &y2);
	printf("%f", sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1)));
	return 0;
}